The present invention relates to a hydraulic control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling a continuously variable transmission having a torque converter with a lockup clutch.
A continuously variable transmission having a torque converter with a lockup clutch is known. The torque converter operates to multiply the torque of an engine, so that the vehicle can be smoothly started. After the start of the vehicle, the torque converter is preferably locked up thereby preventing a loss of power and improving the fuel consumption. Thus, various control systems for controlling the lockup clutch have been proposed in order to effectively use such characteristics.
Japanese Patent Laid Open 55-109854 discloses a control system for a lockup clutch wherein the lockup clutch is engaged depending on the speed ratio of the speed of an input member to the speed of an output member of the torque converter to overcome the inefficiency of the torque converter.
However, if such a control system, in which the lockup clutch is operated regardless of the control of the transmission ratio, is applied to the continuously variable transmission, various problems arise. Namely although rotational speed of a drive pulley increases with the increase of vehicle speed before the start of the changing the transmission ratio (upshift) so as to advance the condition of the torque converter to a condition for a lockup range, the drive pulley speed does not increase after the start of upshifting. Therefore, a problem occurs that the torque converter does not enter into the lockup range. Thus, it is not desirable to control the torque converter irrespective of the transmision ratio. Instead, it is preferable to control the transmission so as to fix the transmission ratio while the torque converter operates to multiply the torque and to start the changing of the transmission ratio and, at the same time, to lock up the torque converter, when the transmission comes into the condition for the lockup and the condition for starting the upshift.
However, in the above-described conventional art, since lockup of the torque converter is determined in accordance with a predetermined speed ratio between the input speed and the output speed, when the difference between the speeds increases although the speed ratio is constant, the lockup shock becomes large.